FIG. 1 is a diagram schematically showing the structure of a prior art digital still camera as disclosed in Japanese patent application publication (TOKKAIHEI) No. 11-331672, and shows the structure of the prior art digital still camera which does not need any frame memory for storing yet-to-be-compressed image data.
In FIG. 1, reference numeral 11 denotes an image pickup lens, reference numeral 12 denotes a shutter which serves as an aperture, reference numeral 13 denotes a CCD which is a photoelectric transducer, reference numeral 14 denotes an analog signal processing unit (or CDS/AGC), reference numeral 15 denotes an AD converter, reference numeral 16 denotes a timing generator (or TG), reference numeral 17 denotes a digital signal processing unit (DSP), reference numeral 18 denotes an image compression unit (or JPEG encoder), reference numeral 20 denotes a flash memory, reference numeral 21 denotes a memory card interface (or PCMCIA I/F), and reference numeral 22 denotes a control unit.
The image pickup lens 11 forms an optical image of light from an object to be shot on a light receiving surface of the CCD 13. While the aperture-shutter 12 regulates the diameter of a light ray traveling from the image pickup lens 11 to the CCD 13 so as to adjust the amount of light applied to the CCD 13, the aperture-shutter 12 closes so as to restrict the exposure time of the CCD 13 when a predetermined time elapses after the CCD 13 has started photoelectric conversion. The CCD 13 includes a matrix of several 100,000 of sets of three types of pixels that are arranged alternately and respond to red (R) light, green (G) light, and blue (B) light, respectively. The CCD 13 converts received light into a charge and accumulates it for every pixel, and outputs stored charges as analog signals.
The analog signal processing unit 14 carries out a double correlation sampling of the output signals of the CCD 13, and also carries out automatic gain processing. The AD converter 15 converts the analog signals input thereto from the analog signal processing unit 14 into equivalent digital signals, respectively, and outputs them to the digital signal processing unit 17.
The timing generator 16 delivers timing signals SH and SV which indicate the timings of horizontal scanning and vertical scanning, respectively, to the CCD 13 via the buffers 16a and 16b. The timing generator 16 also delivers a timing signal SS which indicates a time to sample the output signals of the CCD 13 to the analog signal processing unit 14, and delivers a timing signal SC which indicates a time to convert the output signals of the analog signal processing unit 14 to the AD converter 15.
The digital signal processing unit 17 performs processes, such as white balance correction, shading, interpolation of signals of three colors: R, G, and B, and gamma control, on the output signals of the CCD 13, which are digitized by the AD converter 15, so as to generate image data comprised of a luminance signal and chrominance signals. A set of image data generated by the digital signal processing unit 17 represents a picked-up image of one frame, and can be displayed just as it is.
The image compression unit 18 compresses the image data generated by the digital signal processing unit 17. The image compression unit 18 comprises a discrete cosine transform unit (DCT) 18a for carrying out a discrete cosine transform of the image data output from the digital signal processing unit 17 for every pixel block of a predetermined size (8×8 pixels), a quantizer 18b for quantizing the transformed image data, and a Huffman coding unit 18c for carrying out Huffman coding of the quantized image data.
The flash memory 20 stores the image data compressed by the image compression unit 18. The card interface 21 copies the image data stored in the flash memory 20 to a detachable/attachable memory card on a frame-by-frame basis. Any other equipment according to the JPEG method can reproduce the picked-up image by reading the copied image data from the memory card, and can perform decoding, inverse quantization, and inverse discrete cosine transform on the copied image data.
The control unit 22 adjusts the brightness of the optical image that is formed on the CCD 13 by adjusting the aperture of the aperture-shutter 12. When a release button provided in a non-illustrated control unit is operated and an instruction for starting to store the image in memory is provided, the control unit 22 provides a control signal SO for instructing a start of operation to the image compression unit 18.
In response to the control signal SO from the control unit, the image compression unit 18 provides a control signal ST to the timing generator 16. In response to this control signal ST, the timing generator 16 outputs timing signals SH, SV, SS, and SC to the CCD 13, the analog signal processing unit 14, and the AD converter 15 so as to cause these units to operate at predetermined intervals, respectively. The predetermined intervals at which each of the timing signals SH, SS, and SC is output are set to one eighth of the time required for the image compression unit 18 to compress image data about eight lines.
After outputting the control signal ST, when the time required for the image compression unit 18 to compress image data about eight lines elapses, the image compression unit 18 provides a control signal SP to the digital signal processing unit 17 and provides the control signal ST to the timing generator 16 again. The control signal SP provided to the digital signal processing unit 17 is the one for requesting output of the generated image data about eight lines. The image compression unit 18 compresses the image data that is output by the digital signal processing unit 17 in response to the control signal SP, and each component stops operating until the next instruction is provided thereto by the control unit 22.
The control unit 22 makes the aperture-shutter 12 closed until it is instructed to store an image by actuation of the release button. When actuation of the release button is made, the control unit 22 makes the aperture-shutter 12 open so that the aperture-shutter 12 has a proper aperture. Then, when the predetermined time that is set to about 1/30 seconds elapses, the control unit 22 makes the aperture-shutter 12 closed again. As a result, the CCD 13 has an exposure time as before, and therefore some problems, such as saturation of the CCD 13, and occurrence of fluctuations in the picked-up image due to movement of the object to be shot or camera movement can be prevented.
A problem with digital still cameras is that digital still cameras need a frame memory for temporarily storing generated image data and cannot perform image data compression processing appropriately unless it has any frame memory. In view of this problem, the above-mentioned conventional digital still camera is so constructed as not to need any frame memory for storing yet-to-be-compressed image data.
A problem with the above-mentioned conventional digital still camera is however that since the above-mentioned conventional digital still camera needs no frame memory by stopping actuation of the CCD intermittently, the time required to pick up an image of one frame becomes long.
Furthermore, in order to improve the ease-of-use of image compression processing, a measure of predetermining the number of compressed images which can be recorded in a recording medium is typically taken. For this reason, a storage capacity assigned to an image of one frame is fixed regardless of the type of the image, and it can be assumed that any image data is compressed data having an amount that is close to a certain amount, but does not exceed the certain amount. Such a method of compressing data is called constant rate control. It is generally necessary to dynamically change parameters for image compression processing, such as the contents of a quantization table for use with quantization processing, and to repeat the compression processing until compressed data having an amount equal to or less than the certain amount, in order to perform constant rate control, because the size of compressed image data changes greatly depending upon characteristics of the image data. However, the above-mentioned conventional digital still camera cannot perform constant rate control because it does not have any means for dynamically changing the parameters for image compression processing.
For example, Japanese patent application publication (TOKKAI) No. 11-234669 discloses a method of extracting a high frequency component contained in a digital signal of an image using a high-frequency component extraction circuit, and controlling compression parameters for use with image compression processing based on extraction results, as a method of performing constant rate control.
However, in most cases, a digital still camera changes its settings, such as the operating state and charge storage time of a CCD thereof, and the amount of light incident upon an aperture, when switching between a mode in which image data is always displayed on a liquid crystal display in order to check an image of an object to be shot, and another mode in which an image of the object to be shot is picked up and recorded in a recording medium in response to actuation of a shutter. This is because two purposes of improving the operability of the digital still camera by reducing the amount of transfer of image data and raising the cycle of updating of images at a time of allowing users to check an image of an object to be shot, and increasing the amount of transfer of image data and recording a fine image at a time of recording images are achieved. Since the size of an image to be shot changes depending upon digital zoom processing at a time of allowing users to check an image of the object to be shot and at a time of picking up an image of the object to be shot, two or more patterns also exist in a switching between operation modes of the CCD. A problem is therefore that when constant rate control is performed in such a switching, it is necessary to control the compression parameter by using a high-frequency component extraction circuit or the like while keeping the settings of the CCD and the aperture and therefore the time required to record a picked-up image in a recording medium after a shutter is actuated becomes long.
Another problem is that since the time required to record a picked-up image in a recording medium after a shutter is actuated becomes long, when picking up an image of an object to be shot in rapid motion, there is a large difference between an image of the object to be shot that is formed onto the CCD when a shutter is actuated and an image actually recorded in a recording medium.
The present invention is made in order to solve the above-mentioned problems, and it is therefore an object of the present invention to provide an image pickup apparatus and an image pickup method that can perform constant rate control without lengthening the time required to record a picked-up image in a recording medium even when changing settings, such as the operating state and charge storage time of a CCD, and the amount of light incident upon an aperture, in order to switch between a mode in which image data is always displayed on a liquid crystal display to allow users to check an image of an object to be shot, and another mode in where an image of an object to be shot is picked up and recorded in a recording medium in response to actuation of a shutter.